The present invention relates to a method and apparatus for soldering additional components to a printed wiring assembly without melting the solder used to attach components mounted to the printed wiring assembly in prior operations.
Components are often attached to a printed wiring board to form a printed wiring assembly by infrared soldering, an apparatus for which is shown in Furtek U.S. Pat. No. 4,654,502. In infrared soldering, components are applied to the board, and quantities of solder are also applied to the board in the regions where solder joints are desired. Then, the assembly is heated by infrared radiation to "reflow" or melt the solder to form solder joints. It is also known to use combinations of infrared, recirculating forced hot air heating, and focused forced hot air heating to reflow solder on an entire board, as illustrated in Bahr et al U.S. Pat. No. 4,771,929.
Sometimes it is necessary to attach components to a printed wiring assembly after a previous soldering operation has been performed in which some components have already been attached to the printed wiring board. Whenever such a second soldering step is required, joints where solder has already been melted (reflowed) must be prevented from undergoing a second reflow of their solder, since a second reflow would subject all the previously attached components to dislodging or realignment as well as to a second dose of, life-reducing heat. Such reflowing of the joints is always undesirable, but is particularly undesirable in constructing circuits for military applications and is generally prohibited by military specifications. Thus, if reflow is to be avoided, standard high-speed production soldering techniques such as infrared wave soldering or production oven soldering cannot be used for attaching components to a critical printed wiring assembly if even one component has already been attached to the board.
In the prior art, soldering subsequent to a first soldering operation on a given military specification board has frequently been accomplished using manual methods which are extremely time consuming and require large amounts of rework. In another known method, taught by Papadopoulos et al. U.S. Pat. No. 3,710,069, heat is applied to areas which are to be reflowed while applying fluid cooling to areas which are not to be reflowed. Papadoupoulos e al. also indicates that solders with different melting points can be used on different areas of the circuit board. In this method, the first components are attached with a first, higher temperature solder, and later components are attached at temperatures lower than the first solder temperature with a second, lower temperature solder. This method disadvantageously requires precise temperature control to avoid accidental reflow of the higher temperature solder.
In another known method, as shown in Entwistle et al. U.S. Pat. No. 4,725,716, components are infrared soldered to the first side of a board, and a heat sink is applied to the first side to prevent reflow of the solder on the first side while components are infrared soldered to the other side.
Another known method, as shown in Meyen et al. U.S. Pat. No. 4,160,893, uses a specialized custom machine which holds a single chip in position on a board while applying localized heating controlled by a thermocouple. This machine would require extensive programming for each type of board to be soldered, and might not be adapted to attach all types of components.
While these known methods may be adequate in certain specialized applications, none provides a simple, reproducible, low-cost, and generalized method of attaching components to any location on a printed wiring assembly while protecting previously attached components from heat and reflowing.
Shielding has been also been used for various purposes in infrared soldering. Costello U.S. Pat. No. 3,718,800 shows an apparatus which is placed on a board and has highly reflective surfaces that direct infrared radiation to specific desired regions, thus preventing the application of radiation to other regions. The Costello apparatus also shields the component being soldered to the board from infrared radiation. Unfortunately, since the Costello apparatus encloses infrared radiation in a desired area rather than shielding all other areas of the board, it is not compatible with a production oven apparatus.
Clearly, there is a need for an improved method and apparatus for soldering components to a circuit board subsequent to a previous soldering operation which does not damage the previously-made solder joints, and which is easily applicable to any circuit board configuration, and which is compatible with a production oven or infrared wave soldering so that large amounts of the circuit boards may be quickly mass-produced.